Qt OpenGL memory leak

A have a strange behavior in my Qt OpenGL widget based application.
I just define a widget (inherit from QGLWidget) and in the paintGL function draw large number of 2D lines (1000 x 1000 for test)
A define a timer wich tick updageGL slot each 20 ms
The memory takes 10mo each second until process reach 1Go and then memory usage drop to 20mo and restart growing, ...

If, after painting my lines, I just put an instruction like glCallList () (which just draw 1 line), there is no more memory leaks (and all my previous lines are draw !!!)

I have the same behavior with QT4.8.2 and VS 2008 and Qt5.0.1 with VS 2010.

In fact, all code is now in display lists, so no more problem :)
But I want to understand why the memory increase with this simple drawing.
A can have this problem in another situation so if i can understand with preformances are so strange it will be helpfull